The present invention relates generally to reference generator circuits, and more particularly relates to a temperature-dependent reference generator suitable for large temperature-variation applications.
In the design of various analog and digital circuits, it is often necessary to establish a temperature-independent bias reference within the circuit. This stable bias reference can be either a voltage or a current, although voltage references are most often employed because they are generally easier to interface with other functional sub-circuits. In the case of a voltage reference, the primary emphasis is not on low output impedance, as it is in the case of a voltage source, but rather emphasis is on the temperature stability of the voltage level.
The basic principle of temperature compensation typically involves the use of two temperature-dependent sources, each source having a predictable and opposite polarity temperature drift. One or both sources are then scaled by a temperature-independent scale factor such that when the two temperature-dependent sources are added together, the effects of the two opposite polarity temperature drifts are made to substantially cancel. The resulting reference source will thus ideally exhibit a nominally zero temperature coefficient (TC) voltage or current level.
Current source circuits having an output which is proportional to absolute temperature (IPTAT) are well known and widely used for temperature compensation and temperature sensing to obtain either temperature-dependent or temperature-independent biasing. However, since IPTAT is only proportional to absolute temperature, the maximum current variation that can be generated at any given temperature T is xcex94T/Tnom, where xcex94T is the temperature range of operation, in degrees Kelvin (K), and Tnom is the nominal operating temperature in degrees K. Thus, for example, at 300 degrees K, an IPTAT Current source can have a maximum current variation of 33% in a range from 300 degrees K to 400 degrees K. In many temperature compensation and temperature sensing applications, however, a current variation greater than the absolute temperature variation is required.
Accordingly, there exists a need for a reference source capable of providing an output having a temperature variation greater than the absolute temperature variation.
The present invention provides a temperature-dependent reference generator having an output variation greater than an absolute temperature variation. Using the reference generator described herein in accordance with the present invention, circuits having a relatively high temperature dependency can be easily compensated. Moreover, the reference generator is suitable for temperature sensing with large temperature dependency without requiring a high supply voltage.
In accordance with one aspect of the invention, a reference generator having a temperature-dependent output variation that is greater than an absolute temperature variation includes a first source and a second source, the first source generating an output proportional to absolute temperature (PTAT). The second source generates an output having a temperature coefficient less than or equal to zero. The reference generator further includes a subtraction circuit coupled to the first and second sources, the subtraction circuit subtracting the output of the second source from the PTAT output and generating the temperature-dependent output having a variation greater than an absolute temperature variation.
In accordance with another aspect of the invention, a method of generating a temperature-dependent reference output having a variation greater than an absolute temperature variation comprises the steps of generating a first output having a variation that is proportional to absolute temperature (PTAT) and generating a second output, the second output having a temperature coefficient that is less than or equal to zero. The method further includes the step of subtracting the second output from the first output to generate the temperature-dependent reference output having a variation greater than an absolute temperature variation.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.